1. Field of the Disclosure
The present disclosure relates generally to the production of polyolefin foams for applications in absorbent materials.
2. Background Art
Absorbent materials are used in many personal care articles ranging from baby diapers to hygiene pads for adult incontinence and feminine needs. The efficacy of absorbent articles is dependent on several properties of the core absorbent material including high void volumes, hydrophilicity, wet resiliency, speed of absorption, and the ability to maintain void volume when wet. In diapers, for example, the absorbent core must quickly acquire fluids, distribute it in the void space, and act as a temporary reservoir until the super absorbent polymer (SAP) can absorb and hold the fluid.
The properties of absorbent materials are generally affected by the method with which they are deposited on their supporting surface. For example, wet laid materials generally suffer from high density due to the planar arrangement of the fibers. Air laid materials tend to have high bulk, but are limited in their stability and resiliency in addition to limitations of porosity. Traditional fluff pulp and creped tissue offer high void volumes and are hydrophilic, but collapse when wetted. The development of absorbent foams has proven promising in addressing some of these shortcomings.
Foams and foam materials, which can exhibit high absorbency properties, are generally made from low density elastomers, plastics, and other materials with various porosities. There are six basic types of foams and foam materials: open cellular, closed cellular, flexible, rigid, reticular, and syntactic. Open cellular foams have interconnected pores or cells and are suitable for filtration applications. Closed cellular foams do not have interconnected pores or cells, but are useful for buoyancy or flotation applications. Flexible foams can bend, flex or absorb impacts without cracking or delaminating. Reticular foams have a very open structure with a matrix consisting of an interconnecting network of thin material strands or struts. Rigid foams feature a matrix with very little or no flexibility. Syntactic foams consist of rigid microspheres or glass micro-balloons held together by a plastic or resin matrix.
Foams with varied properties described above are commonly found in personal care products. One method of generating foams is the introduction of air into an aqueous dispersion of polymer particles in the presence of foam stabilizing surfactants via a frothing process. Frothing may be continued until a desired foam density is obtained. At this point the foam may be laid on a substrate or conveyor belt for drying. An example of a polyolefin froth foam (PFF) made according to such a method is shown in FIGS. 1a and 1b, an SEM image of a cross section (FIG. 1a) and surface (FIG. 1b) of a prior art polyolefin froth foam. As shown in FIG. 1a these foams are open cell from surface to surface. That is, the foams have a continuous reticulated open cell morphology that includes small cells on the surface and larger cells toward the middle of the cross section depicting a sponge like morphology. The capillary pressure created by this type of morphology provides the foams with some of the fluid absorption properties desired for hygiene personal care products, such as insult absorption and distribution or wicking. However, limitations on the speed at which the fluid is absorbed (insult rate) exist for these foams. When an insult rate is too low, fluid may overflow the foam surface before all of the fluid insult is absorbed into the foam pad.
Additionally, because the capillary force holding the fluid in the small surface cells of the cellular foam is typically stronger than the osmotic absorption pressure of the SAP, the foam may retain at least a portion of the fluid, preventing absorption by the SAP. This results in an absorbent core having fluid available for rewetting skin when a load is placed on the core.
Another challenge in the formulation of frothed type foams is inconsistent and undesired foam collapse during the drying process, thus making the properties of the foam difficult to control. Fiber-laden foams have proven utility in reducing the degree of collapse during the drying process. U.S. Pat. Nos. 6,261,679 and 6,603,054, both issued to Chen et al., describe methods of making absorbent materials having a base material of cellulosic or other similar fibers. Both the '679 patent and the '054 patent disclose that these hydrophilic fibers should comprise the predominant structural component of the absorbent foam, comprising up to 98% by weight of the foam's mass. A polymer bonding agent that acts as a glue for the strut like fibers largely comprises the remaining 2-10% mass of the foam.
Accordingly, there exists a continuing need for absorbent articles with increased rates of absorption. In the process, it is desirable to maintain high void volumes and increase the surface openings to reduce capillary retention of fluid.